A poo moon is what happens when someone sticks their finger up someone's ass and there is poo that lodges into the nail at the tip of the finger forming a crescent shape.

The poo moon is a method to test to see if the coast is clear for anal entry. One sticks the finger in the ass, pulls it out and looks for poo that may have lodged into the cresecent part of the nail at the tip of the finger. This is called the poo-moon and is a general indication that anal entry is not at this time deemed a wise thing to do.